The present invention relates to a projection-type liquid crystal display device wherein control of voltage applied to a liquid crystal panel, the operating capacity of cooling fans or the like is carried out on the basis of values detected by a temperature detector.
FIG. 25 is a structural diagram schematically showing a projection-type liquid crystal display device according to prior art, as disclosed in Japanese Utility Model Kokoku Publication No. H7-9136 (i.e., No. 9136/1995), wherein a luminous flux (shown by arrows on dot-dash lines) emitted from a light source 43 within a case 42 passes through condenser lens 44, impinges on a liquid crystal panel 45 and is modulated, after which it is enlarged and projected onto a screen 51 by a projection lens 46. The case 42 is further provided with cooling fans 47 and 48, which cause air to flow in a direction W (shown by arrows on solid lines), thereby cooling the interior of the case 42. Reference numerals 49 and 50 represent temperature detectors such as thermistors.
When, however, the projection-type liquid crystal display device according to the above-described prior art is used in a low-temperature environment, the liquid crystal panel 45 does not reach the optimum operating temperature, thereby raising problems, which include slower rate of response, lower moving picture resolution, and, when changing to another image, superimposition of a residual image.
To overcome these problems, it may be considered possible to make use of a liquid crystal panel made of liquid crystal polymer composite (LCPC) material, but it has been found that, depending on liquid crystal panel temperature, it is not possible adequately to overcome the problem of hysteresis in which a residual image is superimposed when changing from one still picture to another.
The projection-type liquid crystal display device according to the above-described prior art had the further problem that, due to changes in the liquid crystal panel temperature, the characteristic curve of the applied voltage vs. light transmittance of the liquid crystal panel would shift and the slope of the characteristic curve is changed, resulting in degradation of the brightness and contrast of the projected image.
Further, in a projection-type liquid crystal display device in which red, green and blue luminous fluxes are modulated, respectively, by three liquid crystal panels and these luminous fluxes are then synthesized to project a color image, a problem arose with respect to the tint of the color image due to difference in the temperatures and color characteristics of the various liquid crystal panels.
FIG. 26 through FIG. 28 are graphs showing the results of measurements of the input signal vs. chroma saturation characteristics for red luminous flux (R), green luminous flux (G) and blue luminous flux (B) in a prototype projection-type liquid crystal display device. From FIG. 26 through FIG. 28, it may be seen that the red and blue luminous fluxes are more abrupt (that is to say, the slope of the characteristic curves is steeper) than in the case of the green luminous flux. The chroma saturation is obtained by the following method. The common voltage amplitude applied to the liquid crystal panel was fixed in an environment of a certain temperature, measurements were made of the illuminance, x and y chromaticity coordinates and tristimulus values Y, and these were converted to the values in the 1976 L*a*b* uniform sensible color space of the CIE (Commission Internationale de l'Eclairage), which is the uniform color space perceived by the human eye. EQU C=(a* 2+b* 2) (1/2) (1)
In Equation (1), indicates an exponent, and the common voltage amplitude indicates the difference between the upper and lower common voltage peak values when the device is driven by an alternating voltage which causes polarity to be reversed field by field or frame by frame.